The left menu items get a CSS class assigned to them based on their name in the menu. If you are using an advanced module (one that is a subfolder in the fuel/modules folder, then you can add a CSS file at fuel/modules/my_module/assets/css/my_module.css to include that CSS class. The blog does this.
If you are using a simple module, meaning one that just maps to a model and doesn't have a module subfolder with it's own controllers, assets, libraries, helpers, etc., then you can set the 'xtra_css' property in you fuel/application/config/MY_fuel.php file to point to a CSS file you want to load that has that CSS class.
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If you are using a simple module, meaning one that just maps to a model and doesn't have a module subfolder with it's own controllers, assets, libraries, helpers, etc., then you can set the 'xtra_css' property in you fuel/application/config/MY_fuel.php file to point to a CSS file you want to load that has that CSS class.